Some methods for solving the above mentioned problem are already patented by well known companies as Yamaha Corporation and Casio. Other inventors have their own methods for solving above mentioned problem.
U.S. Pat. No. 7,102,072 assigned to Yamaha Corporation describes an apparatus and program which forms an envelope function that follows maximum signal values. The method uses signal envelope points selected at characteristic signal time points to determine signal period.
U.S. Pat. No. 5,619,004 assigned to Virtual DSP Corporation is based on correlation functions that need powerful computers.
Many patents use zero crossing points for signal period calculation. The principle of using a zero crossing point is described in U.S. Pat. No. 4,523,506. For complex signals this method is not appropriate.
Another method for signal period determination is based on peak detector (maximum signal detector) and measuring the time interval between two consecutive detected maximums. This method is good for simple signals; complex signals can have more than one maximum in one signal period.
To overcome problems seen with peak detectors and zero crossing detectors, methods using several reference levels were developed. One such method is described in U.S. Pat. No. 4,217,808, where an amplifier with automatic gain control amplifies an input signal. The output of the automatic gain control amplifier is a signal with equal maximum and minimum signal levels. Positive and negative triggers are then set as scaled down maximums or minimums of the amplified signal. The signal period is then measured as the time between first signal and positive trigger crossing point and second signal and positive trigger crossing point when separated by a negative trigger and negative signal crossing point. The automatic gain amplifier is used and trigger values are constant.
The method described herein amplifies the input signal with constant amplification and value of triggers are changed as the input signal maximum and minimum changes. The method herein also defines fast input signal loss detection and criteria for multiple signal period detection. The method herein measures signal half period duration and based on two sums of half period determines multiple signal period. The method herein defines minimum and maximum trigger values and initial trigger values which helps when input signal amplitude varies in time.
In distinction, U.S. Pat. No. 4,627,323 describes a process where signal period measuring is based on two consecutive signal maximum. U.S. Pat. No. 4,688,464 describes a process where some trigger values are changed over time but they are used to detect positive and negative reference peak while signal period measuring is based on a crossing point between constant triggers and a rising signal edge that leads to a positive reference peak. In U.S. Pat. No. 4,688,464, one trigger is set at zero and second is set a little above zero, such that the two crossing points are separated with one negative reference peak. Two consecutive signal periods are compared, and if the difference is quite small they are accepted as periods.